We've run into an issue with the digital certificates used to sign apps, and this is preventing some phones from installing some apps published during the last couple of days.

We've investigated and determined the issue only affects phones that upgraded to Windows Phone 7.5 from an earlier version of the operating system. It does not appear to impact phones sold with Windows Phone 7.5 preinstalled.

Mazhar further adds that only a "small percentage" of the 100,000 plus apps are affected, which include the more popular apps from New York Times, WhatsApp, and Translator from Bing, all of which recently received updates.

Microsoft is already planning on rolling out a fix for the issue, but until then suggests that If you get an error message while installing or updating an app, to wait for the problem to be fixed.

They also don’t recommend uninstalling any apps you already have on your phone, since it's possible that app(s) won't be able to be downloaded again until the issue is resolved.

Microsoft also opened a Microsoft Answers thread on the issue which you can check out here for more details, and a page for developers that explains the issue in more detail here.